The invention relates to a plasma picture screen with a phosphor preparation comprising a phosphor with a coating.
Plasma picture screens render possible color pictures with high resolution, large screen diagonal, and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes in a grid arrangement. The application of a voltage gives rise to a gas discharge which generates light in the ultraviolet range. This light is converted into visible light by phosphors and emitted through the front plate of the glass cell to the viewer. This process is similar to that in fluorescent tubes, but the smaller dimensions of the individual plasma discharges in a plasma picture screen render the conversion of the electrical excitation energy into visible light much less efficient than in fluorescent tubes.
The phosphors used for plasma picture screens are phosphors which are particularly efficient under UV excitation. Frequently used phosphors are, for example, BaMgAl10O17:Eu (BAM) and (Ba,Sr,Ca)5(PO4)3Cl:Eu (SCAP), which are activated by europium in the bivalent state Eu2+. The rigid conditions obtaining during the manufacture of the plasma picture screens, however, lead to a degradation, i.e. a decrease in the efficacy of the phosphors. Suspensions or printing pastes of the phosphors, to which organic binders and dispersing agents are added, are used for applying a phosphor to a picture screen. The organic binders improve the adhesion of the phosphors to the glass, while the dispersing agents enhance the stability of the suspensions. After the phosphor suspensions have been applied, the auxiliary substances must be removed. This is done by means of a thermal treatment at a few hundred degrees Celsius in the presence of oxygen so as to oxidize the organic compounds. Under these extreme conditions, the activator itself may also be oxidized, from Eu2+ to Eu3+. The presence of Eu3+ in the phosphor particles reduces the light output, because Eu3+ ions trap the excitons and return to the ground state without emitting radiation.
It is an object of the present invention to provide a plasma picture screen with a phosphor preparation comprising a phosphor with a coating which does not degenerate under the manufacturing conditions for plasma picture screens.
According to the invention, this object is achieved by means of a plasma picture screen with a phosphor preparation comprising a phosphor with a coating, characterized in that said coating comprises a phosphate with the formula MPO4, wherein M is a metal chosen from the group Al, Sc, Y, Lu, Gd, and La.
These phosphates form particularly good, closed films with dense sines on the phosphors.
It is preferable that the phosphor in addition comprises Eu2+ as an activator.
It is in particular the Eu2+-containing, blue-emitting phosphors which suffer a degradation as a result of the extreme conditions which obtain during the manufacture of a plasma picture screen.
It may be preferred that the phosphor is an aluminate phosphor.
These phosphors are particularly efficient phosphors under UV or vacuum UV excitation. In addition, the surface thereof is covered with hydroxyl groups which can react with the phosphate anions of the coating material under water separation, providing a chemical bond between the coating and the phosphor. The phosphate coating as a result lies very close to the surface of the phosphors.
The invention also relates to a method of manufacturing a phosphor preparation from a phosphor with a coating of phosphates having the formula MPO4, wherein M is a metal chosen from the group Al, Sc, Y, Lu, Gd, and La, in which method
phosphoric acid and urea are added to an aqueous solution of a metal salt, whose metal is chosen from the group Al, Sc, Y, Lu, Gd, and La,
a phosphor is added to this mixture, and
the suspension is heated until the pH value has reached 7.
MPO4 is slowly precipitated by means of this process, in which the pH value is slowly raised through hydrolysis of urea, and a dense and closed coating of the phosphor is obtained thereby.
The invention further relates to a phosphor preparation comprising a phosphor with a coating which comprises a phosphate with the formula MPO4, wherein M is a metal chosen from the group Al, Sc, Y, Lu, Gd, and La.
This phosphor preparation may also be used as a phosphor in a device which operates with a VUV-emitting plasma such as, for example, a xenon discharge lamp.